I don't keep a journal but sometimes I will write things just to reason through an idea before making it public.

I usually keep a copy in a text or markdown file, so maybe I am actually keeping a journal. For example I have lots of writing from my divorce proceedings, saved in multiple locations.

The main thing for me is to have it easily searchable. I use meaningful keywords in file names. I also use Linux for all my files because it has tools like grep that will find every occurrence of a word or phrase in the whole file system.